Current-limiting fuse precluding overshoot op the let-through
current peak over the fusing current peak



Dec. 13, 1966 CURRENT-LIMITING URRENT Filed July 26, l

FIGZQ E. SALZER FUSE PRECLUDING OVERSHOOT OF THE LET-THROUGH PEAK OVER THE FUSING CURRENT PEAK 5 She FIG. I

ets-Sheet l mac INVENTORt WWW Wm Dec. 13, 1966 E SALZER 3,291,941

CURRENT-LIMITING FUSE PREGILUDING OVERSHOOT OF THE LET-THROUGH CURRENT PEAK OVER THE FUSING CURRENT PEAK Filed July 26, 1965 5 Sheets-Sheet :5

7 5 J ab 7 4 7 4 7 4b 3 INVENTOR M mw Dec. 13, 1966 E. SALZER 3,291,941

CURRENT-LIMITING FUSE PRECLUDING OVERSHOOT OF THE LET-THROUGH CURRENT PEAK OVER THE FUSING CURRENT PEAK Filed July 26, 1965 5 Sheets-Sheet 3 FIG.7

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INVENTOR United States Patent CURRENT-LIMITING FUSE PRECLUDING OVER- SHOOT OF THE LET-THROUGH CURRENT PEAK OVER THE FUSING CURRENT PEAK Erwin Salzer, Waban, Mass., assignor to The Chase- Shawmut Company, Newburyport, Mass. Filed July 26, 1965, Ser. No. 474,748 4 Claims. (Cl. 200-120) This invention relates to current-limiting fuses.

It is one object of this invention to provide currentlimiting fuses having a relatively stable arc voltage.

Another object of this invention is to provide currentlimiting fuses precluding an overshoot of the let-through current peak over the fusing current peak and, by so doing, minimizing let-through fi -dt values.

United States Patent 2,964,604 to P. C. Jacobs et 211., Dec. 13, 1960, Current-Limiting Fuses Having Compound Arc-Voltage Generating Means, is concerned with current-limiting fuses conceived for the purpose of achieving a relatively stable arc voltage. The are voltage of fuses according to the above patent is the sum of two components, i.e. a first component having a high initial rate of rise and decreasing rapidly upon having reached a relatively high peak, and a second component having a relatively low initial rate of rise and decreasing slowly upon having reached a relatively low peak value. The first component of the arc voltage is due to the presence of one or more points of initial break which are exposed to the immediate action of a pulverulent arc-quenching filler, preferably quartz sand. The second component is due to the presence of one or more points of initial break sandwiched between a pair of plates of a synthetic-resinglass-cloth laminate.

The structure of United States Patent 2,964,604 tends to produce arc voltages whose peaks are less than the peaks which would obtain under identical conditions in otherwise identical fuse structures wherein the aforementioned sandwiching plates of a synthetic-resin-glass-cloth laminate are omitted. Therefore the fuse structures of United States Patent 2,964,604 are particularly desirable where the initial arc voltage tends to be too high, and where it is desirable, or at least permissible, to reduce the initial peak value of the arc voltage by providing the synthetic-resin-glass-cloth sandwiching plates disclosed and claimed in the above United States patent.

There are instances Where the highest value of the arc voltage incident to fusion and vaporization of a fuse link tends to be relatively low. In such instance the peak of the let-through current tends to overshoot the peak of the fusing current. This is not objectionable, or permissible, in a number of applications. It is, however, very objectionable, or intolerable, wherever the limitation of let-through fi -dt values is critical. In instances where the highest value of the arc voltage tends to be low, and the let-through current overshoots the fusing current, such overshooting is aggravated in the presence of the structure of United States Patent 2,964,604. In other words, that structure should not be applied in such instances. On the other hand, the problem of stabilization of the arc voltage is almost always present, i.e. whether the highest value thereof tends to be relatively low, or relatively high.

It is, therefore, another object of this invention to provide current-limiting fuses having means for stabilizing the arc voltage which are not predicated on any reduction of the peak value of the arc voltage, and preclude the arc voltage from decreasing too rapidly upon having reached its peak value.

3,291,941 Patented Dec. 13, 1966 For a better understanding of the invention reference may be had to the accompanying drawings in which:

FIGS. 1, 2a-2c, and 3a-3c are diagrammatic arc voltage traces obtained under various conditions;

FIGS. 4 and 4a are diagrammatic current traces obtained under different conditions;

FIG. 5 is a longitudinal section of a fuse embodying the present invention;

FIG. 6 is a section of the structure of FIG. 5 taken along 66 of FIG. 5; and

FIGS. 7 and 7a are schematic diagrams explaining the operation of the structure of FIGS. 5 and 6 Referring now to the drawings, FIG. 1 shows diagrammatically the arc voltage occurring in a current-limiting fuse plotted against time. FIG. 1 refers to a situation wherein the peak P of the arc voltage tends to be relatively high and the decrease of the arc voltage too rapid.

FIGS. 2a and 21: show the two components of a composite arc voltage resulting from the application of the structure of United States Patent 2,964,604 and FIG. 20 shows the sum of the arc voltages of FIGS. 2a and 2b. It is apparent from a comparison of FIGS. 1 and 20 that the application of the structure of United States Patent 2,964,604 resulted in a reduction of the peak of the arc voltage from P to P and in a reduction of the rate of decrease of the arc voltage.

In a situation wherein the peak P of the arc voltage of FIG. 1 is about right, but the arc voltage is decaying too rapidly, the arc .voltage should be modified as shown in FIG. 30. This can be achieved, according to the present invention, by making it the sum of the two components shown in FIGS. 3a and 3b, the structure of FIGS. 5 and 6 yielding these two components.

In FIGS. 4 and 4a i has been applied to indicate an available symmetric short-circuit and I has been applied to indicate the peak of the fusing current of a fusible element or fuse link. The peak of the fusing current is reached at the point of time 1 Where the initial arc voltage is relatively low, the current continues to rise after the time t to a peak I which is reached at the time I and then begins to decrease (FIG. 4). If the initial arc voltage is relatively high, the current does not rise beyond the peak I of its fusing current, i.e. does not overshoot that peak, but begins to decrease instantly at the time t (FIG. 4a). The overshoot condition shown in FIG. 4 may be inherent in a given situation. In such a situation the arc voltage should preferably be increased. Since the structure of United States Patent 2,964,604 tends to reduce rather than to increase the peak of the arc voltage, the application of that structure is not indicated in the situation illustrated in FIG. 4.

Referring now to FIGS. 5 and 6, numenal 1 has been applied to indicate a tubular casing of insulating material closed on the ends thereof by a pair of terminal elements 2 in the form of caps or ferrules. Casing 1 is filled with a purlveru'lent arc-quenching filler 3, preferably quartz sand. A fusible element 4 in form of a copper ribbon or silver rib bon is arranged inside casing 1 submersed in filler 3 and interconects conductively the two terminal elements 2. To this end each terminal element 2 is provided with a slot 2a and each end of fusible element 4 projects into said slot in one of terminal elements 2 and is joined to the respective terminal element 2 by a solder joint (not shown). Washers 5 of asbestos or another reasonably heat resistant material are arranged between the axially outer ends of casing 1 and terminal caps 2, and the ends of fusible element 4 project transverse-1y through slots in said washers registering with the aforementioned slots 2a in terminal elements 2. Fusi'ble element 4 defines five serially related points 40 of reduced cross-sectional area, or of relatively small cross-sectional area. Each of these four points 4a is established by three rectangular perforations arranged along a straight line extending transversely across fusible element t. Fusible element 4 further defines four serially related points 4b of relatively large cross-sectional area, each arranged between two points 4a of reduced cross-sectional area. The axial extent of the points 4a of reduced cross-sectional area is very limit-ed in comparison to the axial extent of the points 4b of relatively large cross sectional area. Each point 4a of reduced crosssectional area forms two point-heat-sources when fusible element 4 is carrying current. The change in cross-section from the points 4a of reduced cross-sectional area to the points 4b of relatively large cross-sectional area is abrupt. The small axial extent of the points 4a of reduced cross-sectional area coupled with the above-mentioned abrupt change in cross-section tend to limit thearc voltage formed at each point of break. Each point of relatively large cross-sectional area is sandwiched between a pair of barrier plates 6 of insulating material, preferably a synthetic-resin-glass cloth laminate. Among such laminates melamine-glassoloth laminates are particularly useful for the purpose in hand. Each pair of barrier plates 6 is secured to fusible element 4 by a pair of eyelets 7 projecting transversely through both plates of the pair and avoiding fusible element 4. Each pair of plates 6 is disposed between points 4a of reduced cross-sectional area and none of the pairs of plate 6 covers any point 4a of reduced cross-sectional area. Since all points of reduced cross-sectional area are fully exposed to the quenching action of the pulverulent filler 3, the initial rate of rise of the arc voltage and the peak of the arc voltage are not affected by the presence of plates 6, i.e. initially the arc voltage is of the same magnitude as in the absence of plates 6. The presence of plates 6 decelerates the decrease of the are voltage after it has reached its peak because of the evolution of relatively cool gases from the plates 6. While it is true that the cooling action of quartz sand 3 upon the portion of the arc path between pairs of plates 6 is less than cooling action in the absence of plates 6 and, therefore, generation of a smaller arc voltage might be expected incident to burn'back in the presence of plates 6 than in their absence, their presence imparts to the voltage of the are burning inside of the gap defined between plates 6 the kind of stability illustrated in FIGS. 3a and 312. Each of these two figures shows the sum of the arc voltages across one of the two pairs of series breaks of the structure of FIGS. 5 and 6.

FIGS. 7 and 7a illustrate diagrammatically the present situation. In FIGS. 7 and 7a are voltage is plotted versus time. At the time are voltage 0A is supposed to prevail across an arc gap. A A A are successive increments of time. Acconding to FIG. 7 the arc voltage is supposed to increase during the increment of time A an amount equal to AB due to backburning in quartz sand and to decrease an amount equal to AC due to protracted heating of the arc gap. Consequently, the arc voltage will be OA+AB-AC at the end of the increment of time A the arc voltage decreasing along curve I. According to FIG. 7a the arc voltage increases during the increment of time A from its initial value 0A an amount equal to AB due to evolution of relatively cool gases from plates 6 which flow into the air gap and the are voltage decreases an amount equal to AC due to protracted heating of the arc gap region. While AB AB, the ratio of AB/AC AB/AC' and, therefore, the decrease of the arc voltage according to curve II occurs at 'a smaller rate than the decrease of the are voltage according to curve I. The are voltage OA may be considered as resulting from a predetermined degree of backburning in quartz sand, curve I referring to continued backtburning in quartz sand and curve II referring tocon tinued backburning between a pair of plates 6 (FIGS. 5 and 6) of a melamine-glass-cloth laminate.

It will be understood that various modifications and changes may be made in the embodiment of the invention which has been illustrated and described herein without departing from the scope and spirit of the invention as defined by the following claims.

I claim:

1. A current-limiting fuse precluding overshoot of the let-through current peak over the fusing current peak comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a pulverulent arc-quenching filler inside said cas- ((1) a fusible element in ribbon-form inside said casing, submersed in said filler and conductive-1y interconnecting said pair of terminal elements, said fusible element defining a plurality of serially related points of reduced cross-sectional area; and

(e) sandwich plate means of a gas-evolving insulating material for said fusible element arranged to sandwich only portions of said fusible element situated between said plurality of points of reduced crosssectional area and to expose all of said plurality of points of reduced cross-sectional area to the immediate action of said arc-quenching filler.

2. A current-limiting fuse precluding overshoot of the let-through current peak 'over the fusing current peak comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a body of quartz sand filling said casing;

(d) a fusible element in ribbon-form inside said casing, submersed in said body of quartz sand and conductively interconnecting said pair of terminal elements, said fusible element defining a plurality of serially related points of reduced cross-sectional area; and

(e) a plurality of pairs of sandwich plates of a synthetic-resin-glass-cloth laminate mounted on said fusible element, said plurality of plates being arranged to sandwich only portions of said fusible element situated between said plurality of points of reduced cross-sectional area and to expose all of said plurality of points of reduced cross-sectional area to the immediate action of said body of quartz sand.

3. A current-limiting fuse for precluding an overshoot of the let-through current peak over the fusing current peak comprising in combination:

(a) a tubular casing of insulating material;

(b) a pair of terminal elements closing the ends of said casing;

(c) a pulverulent arc-quenching filler inside said cas- (d) a fusible element in ribbon-form inside said casing, submersed in said filler and conductively interconnecting said pair of terminal elements, said fusible element defining a plurality of serially related points of relatively small cross-sectional area and said fusible element further defining a plurality of serially related points of relatively large cross-sectional area, each of said plurality of points of relatively large cross-sectional area being situated between two of said points of relatively small crosssectional area; and

(e) a plurality of insulating barriers equal in number to said plurality of points of relatively large crosssectional area, each of said plurality of insulating barriers being mounted on said fusible element at a point thereof intermediate two of said points of relatively small cross-sectional area, said plurality of insulating barriers exposing all of said plurality of points of relatively small cross-sectional area of said fusible element to the immediate action of said arequenching filler.

5 6 4. A current-limiting fuse for precluding an overshoot cross-sectional area of said fusible element and said of the let-through current peak over the fusing current plurality of pairs of plates exposing all of said pluralpeak comprising in combination: ity of points of relatively small cross-sectional area (a) a tubular easing of insulating material; of said fusible element to the immediate action of (b) a pair of terminal elements closing the ends of said 5 said body of quartz sand.

casing; (c) a body of quartz sand filling said casing; References Cited y the Examlllel' (d) a fusible element in ribbon-form inside said cas- UNITED STATES PATENTS ing, submersed in said body of quartz sand and conductively interconnecting said pair of terminal ele- 10 1239876 9/1917 Burnilam 200 120 2,502,992 4/1950 Rawllns et al 200120 X ments, said fusible element defining a plurallty of 2 665 348 1/1954 K0 k 200 120 serially related points of relatively small cross-sec- Zac a 2,892,061 6/1959 Kozacka 200120 1onal area and a plurality of serially related points 2,960,589 11/1960 Salzer 200131 of relatively large cross-sectional area, the change 2,964,604 12/1960 Jacobs et a1 200-120 1n cross-sectional area from each of said plurality 15 3 113 195 12/1963 K ck 200 131 of points of relatively small cross-sectional area to 3238333 1/1966 200 120 each of said plurality of points of relatively large ozac a cross-sectional area being abrupt; and FOREIGN PATENTS (e) a plurality of pairs of plates of a synthetic-resin- 908,452 10/1962 Great Britain gIass-cloth laminate equal in number to said plurality 20 of points of relatively large cross-sectional area, each BERNARD A, GILHEANY, Primary Examiner. of said plurality of pairs of plates sandwiching but one of said plurality of points of relatively large GILSON Assistant Exammer 

1. A CURRENT-LIMITING FUSE PRECLUDING OVERSHOOT OF THE LET-THROUGH CURRENT PEAK OVER THE FUSING CURRENT PEAK COMPRISING IN COMBINATION: (A) A TUBULAR CASING OF INSULATING MATERIAL; (B) A PAIR OF TERMINAL ELEMENTS CLOSING THE ENDS OF SAID CASING, (C) A PULVERULENT ARC-QUENCHING FILLER INSIDE SAID CASING; (D) A FUSIBLE ELEMENT IN RIBBON-FORM INSIDE SAID CASING, SUBMERSED IN SAID FILLER AND CONDUCTIVELY INTERCONNECTING SAID PAIR OF TERMINAL ELEMENTS, SAID FUSIBLE ELEMENT DEFINING A PLURALITY OF SERIALLY RELATED POINTS OF REDUCED CROSS-SECTIONAL AREA; AND (E) SANDWICH PLATE MEANS OF A GAS-EVOLVING INSULATING MATERIAL FOR SAID FUSIBLE ELEMENT ARRANGED TO SANDWICH ONLY PORTIONS OF SAID FUSIBLE ELEMENT SITUATED BETWEEN SAID PLURALITY OF POINTS OF REDUCED CROSSSECTIONAL AREA AND TO EXPOSE ALL OF SAID PLURALITY OF POINTS OF REDUCED CROSS-SECTIONAL AREA TO THE IMMEDIATE ACTION OF SAID ARC-QUENCHING FILLER. 